Circumgalactic Precipitation

Feedback from a central supermassive black hole is an essential component of galaxy evolution models. Without it, those models cannot produce realistic massive galaxies and galaxy clusters. However, the black-hole feedback mechanism remains mysterious. Somehow, accretion of matter onto the central black hole of a massive galaxy becomes precisely tuned so that it regulates radiative cooling and condensation of gas in a volume many orders of magnitude larger than the black-hole's gravitational zone of influence. I will discuss how the required coupling can arise through condensation and precipitation of cold clouds out of a galaxy's circumgalactic medium, and will show how such a feedback mechanism suspends the circumgalactic medium in a state that is marginally unstable to precipitation. The characteristics of that marginal state turn out to have much in common with observations of the multiphase circumgalactic media around massive galaxies.